With regard to electronic devices having communication functions (wireless communication devices), it is constantly desired to downsize the entire devices. In addition, the spread of MIMO (multiple-input and multiple-output) communication systems, which enable increase in capacity by simultaneously using multiple antennas, is leading to an increase in the number of antennas installed in a wireless communication device. For these reasons, reduction in both size and cost of antennas installed in wireless communication devices is strongly desired.
In general, antennas such as dipole antennas and monopole antennas, which have low directivities and are capable of emitting radio waves in a wide range of directions, and inverted L antennas, which are a variation thereof, are used in small wireless communication devices. However, since an antenna of this kind needs to be as large as approximately ¼ to ½ of its wavelength in principle, the downsizing of such antennas has been difficult. In addition, a matching circuit is required for impedance matching between antennas and feeding lines, which causes a problem of an increase in manufacturing cost.
PTL 1 discloses a technique for downsizing an antenna by adding a passive element partially made of a magnetic material to a dipole antenna. The technique described in PTL 1 enables downsizing of an antenna and impedance matching without using any matching circuit, by controlling the distribution of the magnetic lines of force around the antenna by use of a magnetic material. NPL 1 discloses a technique for downsizing a monopole antenna by increasing the effective magnetic permeability by disposing a resonator structure called a split ring resonator near the monopole antenna.